ebiederm@cse.unl.edu (Eric Biederman) wrote:
>
>In reading through 20.1 Allocator requirements [lib.allocator.requirements]
>
>I have come across some wording which I find inconsistent with what is
>in the rest of the standard as I understand it.
>
>X::difference_type
>the type that can represent the difference between any two pointers in
>the memory model.
>
>Which seems to say that the operation
>X::pointer a,b;
>  // initialize a & b to point into different objects
>X::difference t = a - b; // seems to be well defined.
>
>later in
>20.4.1 The default allocator [lib.default.allocator]
>in the definition of the default allocator,
>
>class allocator {
>public:
>  typedef size_t size_type
>  typedef ptrdiff_t difference_type
>
>If I understand the definition of ptrdiff_t from C the result is
>undefined if you take the difference of two pointers which do not
>point into the same object.
>
>So is it undefined or not, if you are using the types supplied with
>the allocators?  What is the intent here?

This is a mathematical confusion in the WP found
in more than one place in the STL specs. Another example
is in the description of the < operator.

Both operator- and operator< need be well defined ONLY
on a set of pairs of iterators such that each pair
constitues a valid range.

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